Electronic Circuits 1: Feedback Circuits

Study Notes

A feedback amplifier is a type of amplifier that consists of two parts: an amplifier and a feedback circuit.
The feedback signal is either out of phase (negative feedback) or in phase (positive feedback) with the input signal.

Negative feedback: feedback signal is out of phase with the input signal, used in amplifier circuits.
Positive feedback: feedback signal is in phase with the input signal, used in oscillator circuits.

The input signal is combined with the feedback signal in a mixer circuit.
The output signal from the mixer is applied to the amplifier block, which has an open-loop gain A.
A fraction of the output signal is returned as a feedback signal, which is multiplied by the feedback network block with gain B.
The gain of the feedback system can be estimated as Af = A / (1 + AB).

Reduces frequency distortion: the gain with feedback is approximately equal to B, which is purely resistive and not dependent on frequency.
Reduces noise and nonlinear distortion: the noise signal gain is reduced by (1+AB), allowing for higher gain and reduced noise signal.
Increases bandwidth: the bandwidth increases for an amplifier with negative feedback.
Gain is stable with feedback: the change in closed-loop gain Af is less than the corresponding percent change in the open-loop gain A.

There are four basic ways of connecting the feedback signal: voltage-series, voltage-shunt, current-series, and current-shunt.
Each connection type affects the input and output impedances differently.

Example 1: calculating the voltage gain of an amplifier with negative feedback.
Example 2: calculating the fraction of output feedback and the percentage fall in gain with feedback due to ageing of components.
Example 3: calculating the change in gain of the feedback amplifier due to temperature.
Example 4: finding the feedback fraction, overall voltage gain, and output voltage of a negative voltage feedback amplifier.